Method of purifying aromatic polycarboxylic acid

ABSTRACT

Crude polycarboxylic acid is slurried in an aqueous medium and the slurry is brought into contact with a metal catalyst while preventing catalyst components thereof from contaminating crystals. Since hydrogenation or decarbonylation of a polymerization inhibitory substance or a substance causing coloration can efficiently proceed at a low temperature while suppressing side reactions, a product having such a quality as to permit direct use thereof as such for polymerization can be obtained with good productivity. Also, since the contact temperature can be lowered, simplification of apparatus and energy saving may be attained.

TECHNICAL FIELD

[0001] The present invention relates to a method of purifying a crudearomatic polycarboxylic acid and, more specifically, to a purificationmethod in which a crude aromatic polycarboxylic acid obtained by liquidphase oxidation of a polyalkyl aromatic hydrocarbon is purified by theremoval of polymerization inhibitory substances and substances causingcoloration therefrom to give a purified aromatic polycarboxylic acidwhich can be used directly as such for the polymerization resulting in ahigh molecular weight, colorless polyester resin, etc.

BACKGROUND ART

[0002] Aromatic polycarboxylic acids are commercially importantsubstances as chemical intermediates. Thus, there is a wide demand foraromatic polycarboxylic acids as raw materials of polyesters,polyamides, polyimides, liquid crystal polymers, etc. which are used forfibers, bottles, films and electronic applications.

[0003] As currently widely industrially used aromatic polycarboxylicacids, there may be mentioned terephthalic acid, isophthalic acid,phthalic acid, trimellitic acid, pyromellitic acid,2,6-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid and3,3′,4,4′-biphenyltetracarboxylic acid.

[0004] Known methods for the preparation of an aromatic polycarboxylicacid include a method in which a polyalkyl aromatic hydrocarbon such asxylene, dialkylnaphthalene, dialkylbiphenyl, tetraalkylnaphthalene ortetraalkylbiphenyl is oxidized with molecular oxygen at a hightemperature and a high pressure in the presence of a heavy metal such asCo or Mn and a bromine compound in an acetic acid solvent, and a methodin which the polyalkyl aromatic hydrocarbon is oxidized with air in thepresence of nitric acid, chromic acid or the like. The aromaticpolycarboxylic acid obtained by the above oxidation reaction containsimpurities such as monocarboxylic acids and aldehydes which areintermediate products of the oxidation reaction, bromine adducts andmetal components which are derived from the catalyst and coloringsubstances having unknown structures.

[0005] As a recent increase of necessity for recycling plastic materialssuch as polyesters, materials are now recycled and reused through, forexample, decomposition of PET bottles. In general, however, aromaticpolycarboxylic acids obtained by the above decomposition containimpurities such as colored substances and foreign matters.

[0006] When the aromatic polycarboxylic acids containing such impuritiesare used as raw materials for the polymerization with diols or diamines,physical and mechanical properties, such as heat resistance, mechanicalstrengths and dimensional stability, of the obtained resins areinferior. Therefore, such aromatic polycarboxylic acids cannot be usedas raw materials for polyesters, polyamides and polyimides. Further,crude aromatic polycarboxylic acids obtained by oxidation are generallycolored yellow or black and cannot be used as such for applicationsrequiring transparency such as bottles and films.

[0007] In this circumstance, as a method of purifying terephthalic acid,for example, a method is widely used in which a crude terephthalic acidis completely dissolved in water as a solvent at a high temperature of260 to 280° C. The solution is then subjected to hydrogenation using apalladium catalyst supported on activated carbon so that impurities suchas polymerization inhibitory substances and substances causingcoloration are reduced. From the resulting solution, terephthalic acidis crystallized. By this method, purified terephthalic acid capable ofbeing directly used as such for polymerization may be obtained (JapanesePatent Publication No. 41-16860).

[0008] The above method is for terephthalic acid which is easily solublein water at a high temperature. In order to improve productivity,however, it is necessary to use a temperature as high as 260 to 280° C.and, accordingly, to use a high pressure. Because such a hightemperature is used, side reactions such as hydrogenation on the nucleusare apt to occur and, further, it is necessary to select materials ofthe apparatus while taking corrosion thereof into consideration.

[0009] In the case of naphthalenedicarboxylic acid andbiphenyldicarboxylic acid, since the solubility thereof in water isabout {fraction (1/10)} of that of terephthalic acid, it is necessary touse much higher temperature than 280° C. in order to conduct the abovepurification method with high productivity. This causes extremedifficulty in practical use.

[0010] Purification of an organic compound is generally performed bydistillation, crystallization, adsorption or a combination of theseoperations. Since aromatic polycarboxylic acids have aself-decomposition temperature which is lower than the boiling pointthereof, the purification by distillation is substantially impossible.Further, since aromatic polycarboxylic acids have poor solubility incommonly industrially used solvents, the purification by crystallizationinvolves difficulties. In particular, since naphthalenepolycarboxylicacid and biphenylpolycarboxylic acid are hardly soluble in varioussolvents, industrially advantageous processes for producing high puritynaphthalenepolycarboxylic acid or high purity biphenylpolycarboxylicacid have not yet been established.

DISCLOSURE OF THE INVENTION

[0011] It is an object of the present invention to provide a method ofpurifying an aromatic polycarboxylic acid capable of efficientlypurifying aromatic polycarboxylic acid, which is difficult to bepurified as described above, to give the aromatic polycarboxylic acidwhich can be used directly as such for the polymerization resulting in ahigh molecular weight, colorless polyester resin, etc.

[0012] The inventors have made an earnest study on a method of purifyingan aromatic polycarboxylic acid which method has such problems asdescribed above. As a result, it has been found that when the aromaticpolycarboxylic acid is contacted with a metal catalyst in the absence ofoxygen, while maintaining the aromatic polycarboxylic acid in a slurriedstate, i.e. at such a temperature that the aromatic polycarboxylic aciddissolved in an aqueous medium exists together with undissolved aromaticpolycarboxylic acid, and while preventing catalytic components fromcontaminating the purified aromatic polycarboxylic acid, impurities suchas intermediate products of the oxidation reaction and coloringsubstances can be removed by hydrogenation or decarbonylation at a lowtemperature while preventing the production of by-products so that thepurification can be achieved with a good productivity to give aromaticpolycarboxylic acid which can be used directly as such for thepolymerization resulting in a high molecular weight, colorless polyesterresin, etc. The present invention has been made on the basis of theabove finding.

[0013] Thus, the present invention provides a method of purifying anaromatic polycarboxylic acid, comprising a step of slurrying a crudearomatic polycarboxylic acid in an aqueous medium and a step of bringingthe slurry into contact with a metal catalyst in the absence of oxygenwhile preventing catalyst components from contaminating crystals.

BEST MODE FOR CARRYING OUT THE INVENTION

[0014] The aromatic polycarboxylic acid used for the purpose of thepresent invention is an aromatic hydrocarbon, such as benzene,naphthalene or biphenyl, to which two or more carboxyl groups arelinked. A method of producing such an aromatic polycarboxylic acid isnot specifically limited. For example, the aromatic polycarboxylic acidmay be obtained by oxidizing a raw material compound obtained byintroducing an alkyl group such as a methyl group, an ethyl group or anisopropyl group and a plurality of functional groups capable of formingcarboxyl groups by oxidation, such as formyl groups and acetyl groups,into the above-mentioned aromatic hydrocarbon.

[0015] As aromatic polycarboxylic acids which are currently industriallywidely used, there may be mentioned terephthalic acid, isophthalic acid,phthalic acid, trimellitic acid, pyromellitic acid,2,6-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid and3,3′,4,4′-biphenyltetracarboxylic acid.

[0016] Formylated compounds produced as intermediate compounds duringthe course of the production of aromatic polycarboxylic acid byoxidation of an aromatic hydrocarbon having a plurality of substituents,such as 4-carboxybenzaldehyde in the case of the production ofterephthalic acid and formylnaphthoic acid in the case of the productionof naphthalenedicarboxylic acid, are impurities which are difficult tobe removed and which act as polymerization inhibitory substances andsubstances causing coloration in the subsequent polymerization stage.

[0017] In the method of the present invention, the above-describepolymerization inhibitory substances and substances causing colorationcontained in a crude aromatic polycarboxylic acid are hydrogenated ordecarbonylated by contact with a metal catalyst in the absence ofoxygen. When the contact with the metal catalyst is carried out in thepresence of hydrogen, the polymerization inhibitory substances andsubstances causing coloration are hydrogenated. When the contact withthe metal catalyst is carried out in the absence of hydrogen and oxygen,the polymerization inhibitory substances and substances causingcoloration are decarbonylated. By this way, the impurities are removed.

[0018] Any metal catalyst may be used as the hydrogenation ordecarbonylation catalyst as long as it has an activity and is hardlydeactivated in the purification conditions. A catalyst having a carrieron which catalytic components are supported is generally used.

[0019] As metals to be supported, there may be mentioned Group 8 metals,namely noble metals such as platinum, palladium, ruthenium, rhodium,osmium and iridium, cobalt and nickel. As the carrier, activated carbonis preferably used for reasons of resistance to aromatic carboxylicacid-containing high temperature aqueous solution.

[0020] The temperature and pressure at which the hydrogenation ordecarbonylation is performed vary with the kind of the aromaticpolycarboxylic acid to be purified, the conditions of the impurities andthe catalyst used and are selected so that the hydrogenation ordecarbonylation of the polymerization inhibitory substances andsubstances causing coloration can be efficiently achieved whilepreventing occurrence of side reactions.

[0021] The present invention is characterized in that the aromaticpolycarboxylic acid is contacted with a metal catalyst while maintainingthe aromatic polycarboxylic acid in a slurried state, i.e. at such astate that a portion of the aromatic polycarboxylic acid is dissolved inan aqueous medium, and while preventing catalytic components fromcontaminating the purified aromatic polycarboxylic acid.

[0022] Thus, at the outset, the temperature at which the hydrogenationor decarbonylation is carried out is so selected that the dissolvedaromatic polycarboxylic acid and undissolved aromatic polycarboxylicacid coexist in the form of a slurry, though the temperature varies withthe kind of the aromatic polycarboxylic acid to be purified.

[0023] For example, since the solubility of terephthalic acid in waterat 230° C. is 6.5 g/100 g, a slurry in which dissolved terephthalic acidand undissolved terephthalic acid coexist is formed when the amount ofterephthalic acid relative to water is beyond the solubility.

[0024] Since the hydrogenation or decarbonylation is carried out for aslurry containing undissolved aromatic polycarboxylic acid rather thanfor a homogeneous aqueous solution, it is necessary to contrive a wayfor using the catalyst.

[0025] Namely, it is necessary to retain the metal catalyst so that thecatalyst and the undissolved aromatic polycarboxylic acid are preventedfrom being mixed together. For example, the catalyst particles can beprevented from being mixed into the purified aromatic polycarboxylicacid by holding the catalyst in a basket through which only the slurryis permitted to pass and by immersing the basket in the slurry.

[0026] The concentration of the slurry is so selected that thehydrogenation or decarbonylation of the purities is not hindered andthat the slurry can be transferred using an ordinary industrial means.

[0027] When the aromatic polycarboxylic acid is contacted with the metalcatalyst in the presence of hydrogen, the hydrogenation can be achievedby injecting hydrogen into water as a solvent in which part of thearomatic polycarboxylic acid is dissolved.

[0028] The hydrogen partial pressure in the hydrogenation is so selectedthat the hydrogenation of the aromatic nucleus of the aromaticpolycarboxylic acid is prevented from occurring at the selectedtemperature as described above but that the hydrogenation of theformylated compounds acting as polymerization inhibitory substances andsubstances causing coloration can efficiently proceed.

[0029] Namely, when hydrogenation proceeds excessively, impurities willincrease. Thus, the hydrogen partial pressure in the hydrogenation ispreferably 0.1 to 3 MPa.

[0030] When the aromatic polycarboxylic acid is contacted with the metalcatalyst in the absence of hydrogen and oxygen, it is necessary tosubstitute the atmosphere in the system with an inert gas such asnitrogen so that oxygen is completely removed.

[0031] In the present invention, the term “in the absence of oxygen” isintended to refer to the state in which the atmosphere in the system issubstituted with an inert gas such as nitrogen so that oxygen completelydisappears in the system, i.e. in which the oxygen content is 1 ppm orless, preferably 0.1 ppm or less. As the inert gas to be used for thispurpose is most generally nitrogen. Argon may be used. Carbon dioxide isnot preferable.

[0032] The residence time varies with the kind of the aromaticpolycarboxylic acid to be purified and the state of the impurities butis so selected that the hydrogenation or decarbonylation can be nearlycompleted. In general, the residence time is 0.5 to 5 h.

[0033] Generally, when the hydrogenation or decarbonylation is nearlycompleted, the mixture is cooled to near room temperature. The crystalsthus obtained are rinsed with warm water, etc. and then dried to obtaina purified aromatic polycarboxylic acid.

[0034] According to the present invention, since a large amount of waterrequired to completely dissolve the aromatic polycarboxylic acid is notused, the volume of the reactor used can be small and the purificationof the aromatic polycarboxylic acid can be carried out efficiently.

[0035] Also, according to the present invention, it is not necessary toheat to a temperature required to completely dissolve the aromaticpolycarboxylic acid. Therefore, devices and utility for heating to ahigh temperature are not needed. Further, it is possible to avoidexcessive hydrogenation or decarbonylation, the elimination bydecomposition of carboxyl groups and the formation of polymerizationinhibitory substances and substances causing coloration which would beotherwise caused by heating to a high temperature. Hence, high purityaromatic polycarboxylic acid can be easily obtained.

[0036] Additionally, according to the present invention, it is possibleto purify an aromatic polycarboxylic acid which is substantiallyimpossible to be purified by distillation because the self-decompositiontemperature thereof is lower than the boiling point thereof and which isdifficult to be purified by crystallization because the solubilitythereof in a solvent is low. Further, it is possible to obtain anaromatic polycarboxylic acid which can be used directly as such for thepolymerization to give a high molecular weight, colorless polyesterresin, etc.

EXAMPLES

[0037] The present invention will be described more concretely by way ofexamples and comparative examples. The present invention is, however,not limited to the examples.

[0038] In the present invention, OD₃₄₀ and OD₄₀₀ which are factorsshowing the degree of containing coloring impurities are measured valuesobtained as follows:

[0039] OD₃₄₀: 2 g of terephthalic acid are dissolved in 25 ml of 2N KOHand the solution is charged in a 50 mm cell. Absorbance at 340 nm ismeasured.

[0040] OD₄₀₀: 1 g of naphthalenedicarboxylic acid is dissolved in 10 mlof 1N KOH and the solution is charged in a 10 mm cell. Absorbance at 400nm is measured.

[0041] The values reflect the amount of coloring impurities andsubstances causing coloration contained in terephthalic acid andnaphthalenedicarboxylic acid. The lower the value, the smaller is theamount of the coloring impurities.

Example 1

[0042] A crude terephthalic acid (150 g) containing 3,500 ppm of4-carboxybenzaldehyde (hereinafter referred to as 4CBA) and showingOD₃₄₀ of 1.0 and 600 g of water were charged in an autoclave equippedwith a stirrer. To the stirrer, two baskets each provided with holes forpassage of a terephthalic acid slurry were attached. In the baskets, 20g of coconut hull activated carbon supporting 0.5% of Pd were contained.After closing the autoclave, a hydrogen partial pressure of 0.2 MPa wasestablished therein. With stirring, the contents in the autoclave wereheated to 230° C. From the solubility of terephthalic acid in water at230° C., the amount of terephthalic acid dissolved in 600 g of water iscalculated as 39 g. Heating was stopped 2 h after the temperature of230° C. had been reached. After cooling to room temperature,terephthalic acid was recovered, rinsed with water at 90° C. and dried.The terephthalic acid thus obtained was found to contain 10 ppm of 4CBAand to show OD₃₄₀ of 0.1.

[0043] The terephthalic acid was polycondensed with ethylene glycol toobtain a polyester. Pellets of the thus formed polyester weretransparent.

Example 2

[0044] A crude 2,6-naphthalenedicarboxylic acid (150 g) containing 2,600ppm of formylnaphthoic acid and showing OD₄₀₀ of 1.0 and 600 g of waterwere charged in an autoclave, similar to that used in Example 1,equipped with a stirrer. To the stirrer, two baskets containing 20 g ofthe same catalyst as used in Example 1 were attached. After hydrogenpartial pressure of 0.2 MPa had been established, the temperature wasincreased to 280° C. with stirring. From the solubility of2,6-naphthalenedicarboxylic acid in water at 280° C., the amount of2,6-naphthalenedicarboxylic acid dissolved in 600 g of water iscalculated as 36 g. Heating was stopped 2 h after the temperature of280° C. had been reached. After cooling to room temperature,2,6-naphthalenedicarboxylic acid was recovered, rinsed with water at 90°C. and dried. The 2,6-naphthalenedicarboxylic acid thus obtained wasfound to contain 10 ppm of formylnaphthoic acid and to show OD₄₀₀ of0.040.

[0045] The 2,6-naphthalenedicarboxylic acid was polycondensed withethylene glycol to obtain a polyester. Pellets of the thus formedpolyester were transparent.

Example 3

[0046] A crude terephthalic acid (150 g) containing 3,500 ppm of 4CBAand showing OD₃₄₀ of 1.5 and 600 g of water were charged in an autoclaveequipped with a stirrer. To the stirrer, two baskets each provided withholes for passage of a terephthalic acid slurry were attached. In thebaskets, 20 g of coconut hull activated carbon supporting 0.5% by weightof Pd were contained. After closing the autoclave, nitrogen was fed sothat the pressure therein was increased to 2 MPa. Then the pressure wasreleased to atmospheric pressure. Such procedures were repeated fivetimes so that oxygen contained in the system was completely substituted.Then, with stirring, the contents in the autoclave were heated to 230°C. From the solubility of terephthalic acid in water at 230° C., theamount of terephthalic acid dissolved in 600 g of water is calculated as39 g. Heating was stopped 2 h after the temperature of 230° C. had beenreached. After cooling to room temperature, terephthalic acid wasrecovered, rinsed with water at 90° C. and dried.

[0047] The terephthalic acid thus obtained was found to contain 10 ppmof 4CBA and to show OD₃₄₀ of 0.16. The terephthalic acid waspolycondensed with ethylene glycol to obtain a polyester. Pellets of thethus formed polyester were transparent.

[0048] Comparative Example 1

[0049] The procedures of Example 3 were repeated in the same manner asdescribed using the same apparatus and raw materials as those in Example3, except that oxygen in the system after charging of the raw materialswas not substituted with nitrogen. As a result, the product was found tocontain 11 ppm of 4CBA and to show OD₃₄₀ of 0.34. The terephthalic acidwas polycondensed with ethylene glycol to obtain a polyester. Pellets ofthe thus formed polyester were slightly colored.

Example 4

[0050] A crude 2,6-naphthalenedicarboxylic acid (80 g) containing 1,400ppm of formylnaphthoic acid and showing OD₄₀₀ of 1.0 and 600 g of waterwere charged in an autoclave, similar to that used in Example 3,equipped with a stirrer. To the stirrer, two baskets containing 20 g ofcoconut hull activated carbon supporting 0.5% by weight of Pd wereattached in the same manner as in Example 3. After closing theautoclave, nitrogen was fed so that the pressure therein was increasedto 2 MPa. Then the pressure was released to atmospheric pressure. Suchprocedures were repeated five times so that oxygen contained in thesystem was completely substituted. Thereafter, with stirring, thecontents in the autoclave were heated to 280° C. From the solubility of2,6-naphthalenedicarboxylic acid in water at 280° C., the amount of2,6-naphthalenedicarboxylic acid dissolved in 600 g of water iscalculated as 36 g. Heating was stopped 2 h after the temperature of280° C. had been reached. After cooling to room temperature,2,6-naphthalenedicarboxylic acid was recovered, rinsed with water at 90°C. and dried. The 2,6-naphthalenedicarboxylic acid thus obtained wasfound to contain 50 ppm of formylnaphthoic acid and to show OD₄₀₀ of0.10. The 2,6-naphthalenedicarboxylic acid was polycondensed withethylene glycol to obtain a polyester. Pellets of the thus formedpolyester were transparent.

[0051] Comparative Example 2

[0052] The procedures of Example 4 were repeated in the same manner asdescribed using the same raw materials as those in Example 4, exceptthat the catalyst-containing baskets were not attached. As a result, theproduct was found to contain 1,350 ppm of 4CBA and to show OD₃₄₀ of 0.9.The terephthalic acid was polycondensed with ethylene glycol to obtain apolyester. Pellets of the thus formed polyester were colored.

Example 5

[0053] The same crude 2,6-naphthalenedicarboxylic acid (150 g) as usedin Example 4 and 600 g of water were charged in an autoclave, similar tothat used in Example 3, equipped with a stirrer. To the stirrer, twobaskets containing 20 g of coconut hull activated carbon supporting 0.5%by weight of Pd were attached in the same manner as in Example 3. Afterclosing the autoclave, hydrogen was fed so that the pressure therein wasincreased to 2 MPa. Then the pressure was released to atmosphericpressure. Such procedures were repeated five times so that oxygencontained in the system was completely substituted. After hydrogenpartial pressure of 0.2 MPa had been established, the temperature wasincreased to 280° C. with stirring. From the solubility of2,6-naphthalenedicarboxylic acid in water at 280° C., the amount of2,6-naphthalenedicarboxylic acid dissolved in 600 g of water iscalculated as 36 g. Heating was stopped 2 h after the temperature of280° C. had been reached. After cooling to room temperature,2,6-naphthalenedicarboxylic acid was recovered, rinsed with water at 90°C. and dried. The 2,6-naphthalenedicarboxylic acid thus obtained wasfound to have a formylnaphthoic acid content below the detectable limitand to show OD₃₄₀ of 0.06. The 2,6-naphthalenedicarboxylic acid waspolycondensed with ethylene glycol to obtain a polyester. Pellets of thethus formed polyester were transparent.

INDUSTRIAL APPLICABILITY

[0054] According to the present invention, by converting a crudearomatic polycarboxylic acid into a slurry in an aqueous medium and bybringing the slurry into contact with a metal catalyst in the absence ofoxygen to carry out the hydrogenation or decarbonylation of thepolymerization inhibitory substances and substances causing colorationwhile preventing catalyst components from contaminating crystals, thetemperature of the purification operation can be lowered. Therefore,side reactions can be suppressed and a product having such a quality asto permit direct use thereof as such for polymerization can be obtainedwith good productivity. Also, simplification of apparatus and energysaving may be attained.

[0055] Accordingly, in accordance with the method of the presentinvention, aromatic polycarboxylic acids which have been hithertodifficult to be purified can be now purified with good efficiency in anextremely industrially advantageous manner.

What is claimed is:
 1. A method of purifying aromatic polycarboxylicacid, comprising a step of slurrying a crude aromatic polycarboxylicacid in an aqueous medium and a step of bringing the slurry into contactwith a metal catalyst in the absence of oxygen while preventing catalystcomponents from contaminating crystals.
 2. The method according to Claim1, wherein said contact with the metal catalyst is performed in thepresence of hydrogen.
 3. The method according to Claim 1, wherein saidcontact with the metal catalyst is performed in the absence of oxygenand hydrogen.
 4. The method according to any one of claims 1 to 3,wherein the metal catalyst comprises a carrier and a Group 8 metalsupported on the carrier.
 5. The method according to any one of claims 1to 4, wherein a basket in which the metal catalyst is held is immersedinto the slurry, so that catalyst components thereof are prevented fromcontaminating the crystals of the purified aromatic polycarboxylic acid.6. The method according to any one of claims 1 to 5, wherein thearomatic polycarboxylic acid is terephthalic acid.
 7. The methodaccording to any one of claims 1 to 5, wherein the aromaticpolycarboxylic acid is 2,6-naphthalenedicarboxylic acid.
 8. The methodaccording to any one of claims 1 to 5, wherein the aromaticpolycarboxylic acid is 4,4′-biphenyldicarboxylic acid.